This invention relates to a high-frequency coaxial cable. The coaxial able of the present invention has multiple layers of insulation formed of polymeric materials surrounding a central conductor, and has electrical shielding enclosing the insulation. An outer sheath covers the electrical shielding.
Cables of this general type are commonly known, and are used generally in high-frequency technology for transmitting analog and digital signals. U.S. Pat. No. 5,817,981 discloses a high-frequency coaxial cable in which insulation surrounding a central conductor comprises two layers that differ with respect to dielectric constant. In U.S. Pat. No. 5,817,981, the dielectric constant of the second layer is greater than that of the first layer, with the first layer being formed of a polyethylene and the second layer being formed of a polyimide.
With increasing miniaturization of technical equipment, however, demands are being placed on coaxial cables that cannot be met through solutions known in the prior art. For instance, modern transmission technology requires lightweight connecting lines having extremely small external dimensions but exceptional electrical transmission properties. Moreover, these transmission properties must also be largely independent of outside environmental influences.
In order to meet these requirements, European patent document EP 0 428 622 B1 teaches the manufacture of a high-frequency coaxial cable insulation formed of polytetrafluoroethylene in such a way that a number of strands of porous expanded polytetrafluoroethylene are wrapped around a central conductor in such a way as to form a uniform insulation. This requires a technically complex manufacturing process. Moreover, further miniaturization down to xe2x80x9cmicro coaxial cablesxe2x80x9d having an overall outer diameter of less than 2 mm encounters significant difficulties.
An object of the present invention is to provide for further improvement in the transmission properties of such micro coaxial cables despite the required minimal external dimensions. A particular object of the present invention is to reduce capacitance of the transmission path as much as possible.
These and other objects of the present invention are achieved by providing individual layers of insulation made of fluoropolymers, with at least an inner first layer that encloses a central conductor comprising a melt-formable fluoropolymer from a melt and an outer second layer consisting of a fluoropolymer that is not manufactured from a melt, the second layer being porous and non-positively connected with the surrounding shield. By using two or more fluoropolymer insulation layers for the insulator of the cable in accordance with the present invention, it is possible to adjust the dielectric constant of the insulation the respective requirements, particularly to set low dielectric constant levels, without having recourse entirely to foam insulation. This permits the manufacture of cables with very small external dimensions.
Examples of fluoropolymers than can be manufactured from a melt, i.e., extruded, are tetrafluoroethylene/hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkylvinylether copolymer (TFA/PFA), and HYFLON MFA fluoropolymer. The inner layer can be made compact or as a foam. The wall thickness of this first layer ranges advantageously from about 0.8 through 0.1 mm, preferably from about 0.3 through 0.2 mm, depending on the intended use of the cable.
The second insulation layer adjoining the first is porous, having a microporous structure, as disclosed in European patent document EP 0 489 752 B1. The wall thickness of this second layer ranges advantageously from about 0.8 through 0.2 mm, preferably from about 0.4 through 0.3 mm, again depending on the intended use of the cable. It is advantageous if the dielectric constant of the first layer is greater than that of the second layer.
For compacting the insulation and for further increasing the flexibility of the cable while maintaining the electrical properties at least unchanged, it is often advantageous to glue the two layers to each other.
Particular advantages arise if, according to this invention, in a two-layer insulation the first layer surrounding the central conductor consists of a fluoropolymer that can be manufactured from a melt and the outer, porous second layer consists of a fluoropolymer that is not manufactured from a melt.
This combination of materials in connection with the shielding connected in a non-positive manner with the porous layer leads to a low-capacitance micro coaxial cable having low tolerance of characteristic impedance, low power attenuation, and low interaction impedance in this transmission means.
Further improvements of the cable in accordance with the present invention are obtained if the outermost porous layer, or in the case of a two-layer construction of the insulation, the outer layer, comprises a one-layer or a multiple-layer lapping made of a porous tape. The term xe2x80x9ctapexe2x80x9d in the context of the present invention includes film. Such tape or films may be, for example, polyester-based porous and/or foam films. However, tapes (films) of polytetrafluoroethylene are preferably used.
A tape of this type is stretched and sintered in order to guarantee the porous character of the tape. In this process, the microporous character of the tape material is important. In order to assure microporosity, the tapexe2x80x94for example comprising a polytetrafluoroethylene manufactured by means of paste extrusion followed by rolling, or a polytetrafluoroethylene modified with no more than 2% by weight of fluoromonomersxe2x80x94is subject to a stretching process with a stretch rate of up to 2000%, preferably from 300% through 1000%. The stretching is generally conducted in the direction of the tape, but it can also be done transversely with respect thereto, for instance if the porosity of the tape or of foil is to be increased.
The mechanical strength of the tape of foil material is increase by means of a sintering process that takes place simultaneously with the stretching process or downstream from the stretching process. The thickness of the stretched and advantageously also sintered tape or corresponding foil is then about 15 xcexcm through 250 xcexcm, preferably 30 xcexcm through 100 xcexcm.
In the case of lapping, for purposes of the present invention it is important that at least its outermost tape layer be connected in a non-positive manner with the surface of the electrical shielding facing toward it. This is achieved, for example, by using a hot-melt-type adhesive. The adhesive can be applied by being sprayed on, for instance, for achieving non-positive connection between a conductive plastic or metal foil, or in a further development of this invention, by using an adhesive-coated metal foil as an electrical shield. Aluminum foil coated with polyester has proven advantageous as a metal foil in this context.
The non-positive connection between the porous outermost layer of the insulation and the conductive shielding is generally achieved during extrusion of the outer sheathing of the cable, owing to its heat content. This is particularly true if, as provided according to the invention, the outer sheathing consists of a fluoropolymer having a correspondingly high melting/extrusion temperature of, for instance, 350xc2x0 C. Such temperatures in the outer area of the cable effect a melting on of the adhesive layer between the porous insulation and the electrical shield. The adhesive then intersperses with the pores of at least the outermost layer of a lapping comprising a stretched foil that serves as a second layer of the cable insulation, for example. When the outer sheathing cools, the shrinkage effect associated therewith, particularly with regard to fluoropolymers, solidly anchors the sheathing to the cable insulation by a multiplicity of adhesion points. This anchoring is permanent, even with regard to large temperature fluctuations and relevant operating temperatures, as well as when the cable is under mechanical stress. Furthermore, therefore, any crimping or wrinkling of a thin aluminum foil, which would necessarily lead to a deterioration of the electrical transmission properties, is avoided. This also applies for micro coaxial cable for transmitting analog and digital signals with correspondingly small external dimensions.
If the heat content of the extruded outer sheathing is insufficient for forming a secure connection between the porous insulation, owing for instance to the amount of extruded mass per unit of length being too small or to the polymer materials used as the outer sheathing having a low melting/extrusion temperature, then an additional heat treatment is recommended following the application of the electrical shielding. This is because a significant feature of the coaxial cable according to the present invention is the mechanically fixed all-surface connection between, for example, a metal foil and the outermost porous insulation layer of the cable.
The shielding of the cable is advantageously structured in two layers. Outward of the above-described adhesive-coated metal foil or metallized plastic foil, an outer layer in the form of a metal wire layer or a braided covering comprising individual metal wires is provided. Outward of that is located the outer sheathing, which can be made from fluoropolymers or halogen-free flame resistant polymer materials or flame resistant, anti-corrosive polymer materials, such as polyolefins, elastomers, or thermoplastic rubber. This two-layer shielding has the advantage of improved shielding properties at the same time as high flexibility of the cable.